An electrodynamometer wattmeter have two types of coils. One coil is fixed and another coil is moving. The fixed coil is connected in series to the circuit whose power consumption is to be calculated. Therefore this fixed coil is often called Current Coil or simply CC. Moving coil is connected across the supply voltage. Due to this current proportional to supply voltage flows through the moving coil. To limit the current through this coil, a resistor is connected in series with the moving coil. Moving coil is called Pressure Coil or simply PC.

Construction of Electrodynamometer Wattmeter

Fixed Coil

Fixed coil or CC is carries the circuit current. It is divided into two halves. Fixed coil is chosen as a current coil as it can be made more massive. Therefore they can easily carry heavier current. Fixed coil is wound with heavier wire. The wire is either stranded or laminated to avoid eddy currentloss. In earlier design of Electrodynamometer wattmeter the fixed coil were designed to carry current of the order of 100A. Due to this the sizes of earlier wattmeter were large. In modern wattmeter, CC is designed to only carry a current up to 20A. Therefore in circuit were power is quite high, Current Transformer shall be used for the measurement of power.

Moving Coil

Moving coil is also called Pressure Coil or PC. It is mounted on pivoted spindle. This coil is entirely embraced by current coil to have flux linkage between CC and PC. For the control of movement spring is used. As discussed earlier in the post, a series resistor is used in PC to limit the current upto 100 mA.

Both moving coil and fixed coils are air cored. The voltage limit of moving coil is limited to about 600 V by the power requirement of voltage circuit since most of the power is dissipated in the series resistor of voltage circuit. Due to this power dissipation in series resistor of PC, heat is generated. Therefore, for higher voltages, PC is designed for 110 V. Therefore for higher voltage Potential Transformer shall be used.

Control

Spring Control is used in such instrument for proving controlling torque.

Damping

Air friction damping is used. The moving system comprises a light aluminium vane which move in a sector shaped box. Electromagnetic or Eddy current damping cannot be used in electrodynamometer wattmeter as it will distort the weak operating magnetic flux.

Scale and Pointer

The scale is mirror type and knife edge pointer to eliminate reading error due to parallax.